Biodiesel fuels produced from vegetable oils have drawn much attention as alternate fuels for fossil fuels and also in terms of low emission of carbon dioxide, and therefore, an increase in demand for them has been expected. Since the production of such biodiesel fuels is accompanied by the formation of glycerin as a by-product, it is required to make effective use of glycerin. An embodiment of making use of glycerin is to use glycerin as a raw material for acrolein.
Acrolein is a useful compound to be used as a raw material of acrolein derivatives such as 1,3-propanediol, 1,2-propanediol, methionine, acrylic acid, 3-methylpropionealdehyde, and water-absorbing resins. Japanese Patent Application Laid-Open (Kokai) No. Hei 06-192147 discloses producing 1,3-propanediol and 1,2-propanediol using acrolein obtained by dehydration of glycerin. Further, Japanese Patent Laid-open Publication (Kokai) No. 2005-213225 discloses producing acrylic acid by gas-phase oxidation of a gas-phase dehydration product of glycerin. Further, WO 2006/092272 discloses obtaining acrolein from glycerin, converting this acrolein into acrylic acid by the heretofore known gas-phase oxidation, and further producing a water-absorbing resin from this acrylic acid by the heretofore known process.
It has been well known since long ago that acrolein can be produced by dehydration of glycerin and also that a solid acid catalyst is used in this production.
Japanese Patent Laid-open Publication (Kokai) No. Hei 06-211724 discloses producing acrolein by dehydrating glycerin in gas phase by bringing a glycerin-water mixture containing from 10% to 40% by mass of glycerin into contact with a solid acid catalyst having an acid strength function H0 of +2 or lower (e.g., a solid acid catalyst obtained by allowing phosphoric acid to be supported on an aluminum oxide carrier) under the condition of from 250° C. to 340° C. The feed amount of glycerin per 1 hour to the catalyst is calculated to be from 80 to 160 g/hr in Examples of this publication.
Further, WO 2006/087083 discloses a process for producing acrolein by gas-phase dehydration of glycerin using a solid strongly acidic catalyst having an acid strength function H0 of from −9 to −18 and describes that the reaction temperature may be preferred to be from 250° C. to 350° C. in the catalytic gas-phase dehydration using a glycerin-water mixture having a concentration of from 10% to 50% by mass and the solid strongly acidic catalyst. The feed amount of glycerin per 1 L of the catalyst in this publication is calculated to be 230 g/hr.
In addition, with respect to a process for producing acrolein using not glycerin but propylene as a raw material, Japanese Patent Laid-Open Publication (Kohyo) No. 2006-521317 discloses a process for producing acrolein by catalytic gas-phase partial oxidation of propylene. The feed amount of propylene per 1 L of a catalyst in this process is from 90 to 160 NL and when this feed amount is converted into the feed amount of glycerin per 1 L of the catalyst, it becomes from 360 to 658 g/hr.
The above respective publications disclose gas-phase reaction of glycerin using a dilute aqueous glycerin solution and suggest using a raw material gas composed of low-concentration glycerin and high-concentration water vapor; however, none of the above respective publications describes that the partial pressure of glycerin gas in the raw material gas affects on the life of a catalyst when the feed amount of glycerin is fixed to be a constant.
When the partial pressure of glycerin is lowered in a catalytic gas-phase reaction using a solid catalyst, there are (a) a method of lowering a load on the catalyst by decreasing the feed amount of glycerin depending on the partial pressure of glycerin; and (b) a method of increasing the space velocity of the entire raw material gas while a load on the catalyst is kept constant by increasing the amount of dilution gas without changing the feed amount of glycerin. In the former method, it is assumed that both product selectivity and catalyst life may be improved, whereas in the latter method, it is assumed that even if product selectivity is improved, the effect on the improvement in catalyst life is small because a load on the catalyst is constant and rather the conversion rate of glycerin may sometimes be lowered.
When the production of acrolein on an industrial scale is assumed, in the gas-phase reaction using, as a raw material, a dilute aqueous glycerin solution as disclosed in the above respective publications, a great amount of energy is consumed to evaporate a great amount of water. Further, since acrolein, which is an aimed product, has a boiling point (i.e., 52° C.) lower than that of water, energy is further consumed for the condensation of all of water vapor produced by evaporating the raw material and water vapor generated by dehydration. In addition, after acrolein is extracted from the aqueous acrolein solution obtained by the condensation, it is required to treat a great amount of wastewater. Accordingly, even if acrolein or acrolein derivatives, which are useful compounds, are produced using, as a raw material, glycerin derived from plants which are said to be earth conscious, a process for producing acrolein, which is accompanied by mass consumption of energy and mass generation of wastes, has a problem of being difficult to say that it makes consideration to the global environment.
Further, when the production of acrolein on an industrial scale is assumed, since the feed amounts of glycerin as disclosed in the above respective publications are small, the productivity of acrolein is low. Accordingly, there is an issue that an apparatus for producing acrolein becomes large, and an improvement in the productivity thereof is desired.
WO 2006/087084 discloses a process for producing acrolein under the conditions of atmospheric pressure, gas phase, and glycerin concentration of 50% by mass or lower (i.e., glycerin concentration of 16 mol % or lower in the raw material gas at the inlet of a reactor). This process uses an aqueous glycerin solution as a raw material and supplies water at an amount greater than that of glycerin to the reactor.
Japanese Patent Laid-open Publication (Kokai) No. 2005-213225 discloses a process for producing acrolein by gas-phase dehydration of glycerin and producing acrylic acid by oxidation of acrolein. In the gas-phase dehydration, the amount of water to be added to glycerin under the conditions of atmospheric pressure and gas phase is 50% by mass or lower. Further, an inert gas such as nitrogen is added to the raw material gas to be supplied to a reactor and the concentration of the inert gas in the raw material gas is 50 mol % or higher at the inlet of the reactor and the concentration of glycerin in the raw material gas is from 10 to 14 mol %.
“Le H. Dao, Reaction of Model Compounds of Biomass-Pyrolysis Oils Over ZSM-5 Zeolite Catalysts, American Chemical Society, 1988, 376, p. 328-341” discloses a process for producing acrolein using glycerin and helium gas but no water. In this literature, the yield (percent yield) of acrolein is not clearly described; however, the yield of acrolein is insufficient.
In general, if the concentration of glycerin is increased, it is possible to improve the productivity of acrolein and make small the size of a reactor for the production of acrolein. For this reason, it is expected that fixed costs, as well as variable costs required for evaporation of the raw material, heating of the raw material, and separation and refining of acrolein, can be saved, and therefore, it is expected that an increase in the concentration of glycerin has many advantages as compared with the case of decreasing the concentration of glycerin.
When low-concentration glycerin gas is used as disclosed in WO 2006/087084, Japanese Patent Laid-open Publication (Kokai) No. 2005-213225, and American Chemical Society, there are concerns about, for example, low productivity of acrolein and an increase in facility cost because of an enlargement of the reactor size. Further, when a low-concentration aqueous glycerin solution is used as described in WO 2006/087084 and Japanese Patent Laid-open Publication (Kokai) No. 2005-213225, there is a problem that a great amount of energy is required to evaporate a great amount of water and a great amount of energy is also required to condense water vapor along with the condensation and recovery of acrolein from a gaseous product obtained. Further, when a noncondensable gas such as helium gas is used as described in American Chemical Society, acrolein is scattered at the time of condensation and recovery of acrolein from a gaseous product, and a loss of acrolein is a probable risk. In addition, it is desired to improve the yield of acrolein and the life of a catalyst.
None of WO 2006/087084, Japanese Patent Laid-open Publication (Kokai) No. 2005-213225, and American Chemical Society describes the effects of carrying out the gas-phase dehydration of glycerin under a reduced pressure condition. Further, these documents contain neither descriptions nor suggestions on the conditions of gas-phase dehydration under which the yield of acrolein is increased, the recovery efficiency of acrolein is improved, and the yield of acrolein is not lowered even if the concentration of glycerin is increased.
As described above, it is desired to improve energy consumption or other problems in the process for producing acrolein. As another problem to be improved in the production of acrolein, there is the reactivation of a catalyst, of which activity has been deteriorated. WO 2006/087083 discloses that carbonaceous substances are accumulated on a catalyst or that the activity of a catalyst is deteriorated with time; however, this publication discloses no reactivation conditions of a catalyst, of which activity has been deteriorated, and makes no reference to the relationship between the temperature of gas-phase dehydration of glycerin and the temperature of catalyst reactivation.
WO 2006/087083 describes that: (1) if the reaction temperature is lowered, the conversion rate of glycerin is decreased, but the selectivity of acrolein is improved; (2) if the contact time of glycerin and a catalyst is prolonged, the conversion rate of glycerin is improved, but the prolongation of the contact time is limited to avoid consecutive reactions and formation of by-products; and (3) the prolongation of the contact time is effective as means for making an improvement in the conversion rate of glycerin when the reaction temperature is low; however, there is no specific description on the relationship between the contact time and yield of acrolein. Further, the relationship between the feed amount of glycerin and the reaction conditions are not disclosed at all, and even if acrolein is produced at the feed amount under the reaction conditions as shown in Examples, the production efficiency of acrolein under these conditions is insufficient from an industrial point of view, and therefore, when these conditions are employed, a great amount of a catalyst and a large-size reaction apparatus are required. That is, from an economical point of view, it is desired to improve the reaction conditions.